Hydraulic Fracturing
Hydraulic fracturing is a well-known process used to recover hydrocarbon fluids such as gases (e.g., natural gas) or liquids (e.g., oil or petroleum) that are otherwise trapped or present in the pores of underground formations. At its most basic level, hydraulic fracturing involves the creation of fractures (e.g., cracks, fissures, etc.) in rock, or underground formations, which will allow hydrocarbon fluids to flow toward a production well. That is, hydraulic fracturing of subterranean formations (sometimes referred to as fracking), provides a pathway for hydrocarbon fluids to more easily move through tight low permeability (micro Darcy) formations (e.g., shale, certain clay sandstones, limestones, etc.) to a production well.
Once a hole is properly drilled for extracting hydrocarbon fluids (i.e., a “wellbore”), fracturing fluids are injected under high pressure into a wellbore to produce fractures that will enhance the flow of hydrocarbon fluids trapped in the subterranean formation. These fracturing fluids typically include a proppant, which is a solid material such as rounded sand grains, bauxite or ceramics that function to “hold” the newly created fractures open after the fracturing pressure is relieved so that the hydrocarbon fluids trapped in the subterranean formation may flow to the intended production well. That is, these “propped fractures” serve as pathways linking a greater proportion of the interior depths of the formation containing the trapped hydrocarbon fluids directly to a production well.
Alternatively, hydraulic fracturing can also involve the injection of prepad, pad and proppant slugs into a properly prepared wellbore under very high pressure.
In either scenario, the key to hydraulic fracturing is the application of high pressure to create fractures in the formation's pay zone (the zone containing the gases and/or liquids) that is then filled with proppant materials to prevent its complete closure after removal of the applied pressure. Sufficient pressure must be applied to exceed the overburden confining pressure of the rock formations above the pay zone.
Once the fractures have been created, a flush fluid is subsequently injected to break gels and sweep residual materials from the wellbore in preparation for hydrocarbon production. As a result, large amounts of fresh or clean water are often required in hydraulic fracturing. Indeed, water management is a serious concern during hydraulic fracturing operations—a single well with multi-stage fracturing may require several million gallons of relatively fresh water to complete the operation in a long horizontal lateral. In arid areas or anywhere where water resources are scarce due to other existing public and commercial needs, acquiring and transporting sufficient water can be a challenge and impose a significant cost consideration.
In addition, after a hydraulic fracturing or stimulation operation is complete, the water used/recovered (i.e., “flowback” or “back flow” water) often contains high levels of total suspended solids (TSS) and total dissolved solids (TDS) with multi-valent cations such as Ca, Mg, Fe, Al, Sr, Ba, Ra, etc. picked up from the formation as well as residual amounts of the chemicals initially added to perform the hydraulic fracturing process. This water must be treated and/or properly disposed, and/or cleaned up sufficiently so that it can be recycled for use in subsequent stimulation jobs on nearby wells. As such, water impurity levels are also a concern in hydraulic fracturing processes.
In an effort to address these water concerns, foamed hydraulic fracturing gained commercial acceptance in the early 1970's, and is used in up to one third of current fracturing applications. In a foamed hydraulic fracturing process, gases such as carbon dioxide (CO2) and nitrogen (N2) are used with appropriate surfactants, polymers and crosslinking agents to create stable highly viscous foams to assist in transporting proppants into the fractures. High quality foams (>60%) can help reduce the amount of water that is required to complete a fracturing operation. In addition, the use of foams also reduces exposure of sensitive formations to water that could result in clay swelling, or plugging. Foams contribute to faster flowback and make cleanup of the formation and wellbore easier in preparation for production.
However, an issue with foams is that they are shear thinning non-Newtonian fluids and thus, the viscosity of the foamed fluid falls off significantly with shear rate. Therefore, the high fluid shear rates associated with high liquid injection rates can lead to elevated leakage of the foam into existing cracks and fissures, making such foams less effective in propagating fractures into new areas. For this reason, foams are generally limited to only very tight formations. Higher mesh sands are often used to compensate for this which, however, results in lower ultimate productivity. Also, foams are best at suspending low concentrations of proppants, and consequently yield narrower propped fractures once the pressure is relieved. As such, foamed hydraulic fracturing often results in less overall productivity than traditional liquid (“slickwater” or gelled) hydraulic fracturing techniques.
Therefore, there remains a need for an improved hydraulic fracturing method which will reduce the amount of water required and reduce the amount of back flow impurities while maintaining a high level of hydrocarbon fluid production.
Pipelines
Over time, asphaltene deposits and/or waxy deposits have been known to buildup in hydrocarbon fluid pipelines as a result of low temperatures making them less soluble in the native hydrocarbon fluid. The buildup of these deposits can result in a reduced flow of the hydrocarbon fluid. In situations in which the pipeline is hard to reach, this can make the removal of these deposits difficult.
For example, subsea pipelines are pipelines that are laid down on the bottom of the sea or ocean bed to transport hydrocarbon fluids such as crude oil from offshore platforms to on-shore collection facilities. Due to the relatively low subsea temperatures and the use of un-insulated pipes being often employed, there is a tendency for waxy, or asphaltene, materials contained in the crude oil to precipitate out of solution and deposit on pipeline walls. Over time, this buildup can slow flow reducing operating efficiency, or even eventually block the pipeline. The location of the subsea pipeline at the bottom of the sea/ocean makes it very difficult to remove or clean up the buildup of asphaltene or waxy deposits.
The conventional method used to clean subsea or conventional pipelines have been the use of pipeline intervention gadgets (or devices) also known as “pigs” to mechanically clean the pipeline. However, pigs require pipelines of constant diameter just larger than the pig itself and the pipeline cannot contain any mechanical obstructions or certain types of valves (such as butterfly valves or reduced portball valves) that would obstruct the pig from moving through the pipeline.
As such, there is a need for an improved method to reduce the buildup of asphaltene or waxy deposits in subsea or conventional pipelines.